Method for producing an electromagnetic radiation-emitting semiconductor chip and a corresponding electromagnetic radiation-emitting semiconductor chip

ABSTRACT

In a method of fabricating a radiation-emitting semiconductor chip based on AlGaInP, comprising the method steps of preparing a substrate, applying to the substrate a semiconductor layer sequence comprising a photon-emitting active layer, and applying a transparent decoupling layer comprising(Ga x (In y Al 1−y ) 1−x P wherein 0.8≦x and 0≦y≦1, it is provided according to the invention that the substrate is made of germanium and that the transparent decoupling layer is applied at low temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/DE2003/002786, filed Aug. 21, 2003, which claims the benefit ofGerman Patent Application Serial No. 10239045.2, filed on Aug. 26, 2002.The contents of both applications are hereby incorporated by referencein their entireties.

BACKGROUND OF THE INVENTION

In this context, materials based on InGaAlP include in particular allmixed crystals whose composition falls under the formula(Ga_(x)(In_(y)Al_(1−y))_(1−x)P, wherein 0≦x≦1, 0≦y≦1 and x+y≦1.Electromagnetic-radiation-emitting semiconductor chips based on AlGaInPinclude all semiconductor chips in which the semiconductor layersequence in which an electromagnetic-radiation-generating layer isdisposed comprises at least a substantial proportion of InGaAIP-basedmaterial and the properties of the radiation emitted by thesemiconductor chip are at least substantially determined by theInGaAIP-based material.

This material based on InGaAlP need not necessarily have a compositionthat is mathematically exactly that of the above formula. Rather, it caninclude one or more dopants and additional constituents.

The AlGaInP material system is very attractive for use in light-emittingdiodes (LEDs), since its bandgap can be adjusted by varying the Alcontent over a broad range of 1.9 to 2.2 eV. This means that LEDs can bemade from this material in the color range of red to green.

To fabricate such LEDs by epitaxy, a substrate is needed on which thevarious semiconductor layers in the sequence can be deposited insofar aspossible in monocrystalline form. Such a substrate for the epitaxy ofAlGaInP-based LEDs should meet the following conditions:

-   -   it should have a lattice constant that enables the material        systems AlGaInP and AlGaAs to be deposited in monocrystalline        form,    -   it should remain sufficiently solid at the process temperatures        used, and    -   it should be available commercially in sufficiently good        quality.

All the aforesaid conditions are met by GaAs substrates. GaAs isconsequently used throughout the world as a substrate for AlGaInP LEDs.From the standpoint of economical LED manufacture, however, GaAssubstrates have the disadvantage of being expensive and containingarsenic. Other substrate materials either have a high lattice mismatchor are not adequately suited for the usual process steps.

SUMMARY OF THE INVENTION

The object of the invention is, therefore, to provide a method of thespecies cited at the beginning hereof that permits the technicallysimple and low-cost fabrication of a radiation-emitting semiconductorchip based on AlGaInP.

This object is achieved by means of a method having features describedherein Further advantageous embodiments and improvements of the methodwill emerge

An electromagnetic-radiation-emitting semiconductor chip that can befabricated according to the method of the invention is also describedherein. Advantageous embodiments and improvements of the semiconductorchip will also emerge in the following description.

An electromagnetic-radiation-emitting semiconductor chip that can befabricated according to the method of the invention constitutes thesubject matter of Claim 9. Advantageous embodiments and improvements ofthe semiconductor chip of the invention form the subject matter ofdependent Claims 10 and 11.

The invention concerns a method of fabricating anelectromagnetic-radiation-emitting semiconductor chip based on AlGaInP,comprising the method steps of: preparing a substrate; applying to thesubstrate a semiconductor layer sequence comprising a photon-emittingactive layer; and applying a transparent decoupling layer, particularlya decoupling layer comprising (Ga_(x))In_(y)Al_(1−y))_(1−x)P, wherein0.8≦x and 0≦y≦1. The invention also concerns anelectromagnetic-radiation-emitting semiconductor chip based on AlGaInP,comprising a substrate, a semiconductor layer sequence applied to thesubstrate and comprising a photon-emitting active layer, and atransparent decoupling layer disposed on the active layer and comprisingGaP.

It is provided according to the invention, in a fabrication method ofthe species cited at the beginning hereof, that the substrate issubstantially composed of germanium and that the transparent decouplinglayer is applied at low temperature. Germanium has a lattice constantthat is readily tolerated with the material systems AlGaInP and AlGaAsand is available commercially in high quality. Moreover, the price of agermanium substrate is only about half the price of a GaAs substrate,resulting in great savings potential for the production process.

The lower thermal stability of germanium compared to GaAs is taken intoaccount by the fact that the especially critical step of growing thegallium-phosphide-containing transparent decoupling layer is performedat a low temperature at which the germanium substrate still has adequatesolidity and a low vapor pressure.

In a preferred embodiment of the method of the invention, it is providedthat the transparent decoupling layer is applied in the form of aphosphorus source, using tertiary butyl phosphine (TBP, (C₄H₉)PH₂).Conventional LEDs based on AlGaInP typically involve the use of alight-decoupling layer of GaP which is deposited epitaxially, usingphosphine (PH₃), at a temperature above 800° C. Such reactortemperatures are too high for processes involving germanium substrates.The use of tertiary butyl phosphine as a phosphorus source, however,makes it possible to deposit a high-quality light-decoupling layer atmuch lower process temperatures.

In particular, it is especially advantageous to apply the transparentdecoupling layer at a temperature below 780° C., preferably below 750°C.

It is particularly preferred if the transparent decoupling layer isapplied at a temperature of about 700° C.

It is also frequently advantageous to apply the transparent decouplinglayer using trimethyl gallium as a gallium source.

Given a typical lateral dimension for the active layer of A=250 μm, thethickness of the decoupling layer is then selected to be between about 1μm and about 10 μm, preferably between about 2 μm and about 10 μm.

In an advantageous embodiment of the method of the invention, thetransparent decoupling layer is grown by metal-organic vapor-phaseepitaxy (MOVPE).

During the growth of the transparent decoupling layer, the V:III ratiois advantageously adjusted to a value of 5 to 20, preferably about 10.

Further advantageous embodiments, features and details of the inventionwill emerge from the dependent claims, the description of the embodimentexample and the drawing.

Further advantages, preferred embodiments and improvements of theinvention will emerge from the following explanation of an embodimentexample in conjunction with the drawing. Only the elements essential toan understanding of the invention are represented.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a schematic diagram of a sectional view of aradiation-emitting semiconductor chip according to an embodiment exampleof the invention.

FIG. 1 is a sectional view of an AlGaInP-based LED chip 10 generallydenoted by 10, shown in schematic representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The LED chip 10 comprises a germanium substrate 12 on which asemiconductor layer sequence 14 is formed. In the embodiment example,the semiconductor layer sequence 14 is a double heterostructurecomprising an active, photon-emitting, AlGaInP-based n-type layer 22enclosed by an AlGaInP-based n-type confining layer under the activelayer an AlGaInP-based p-type confining layer over the active layer.Structures and layer sequences of this kind are known to the skilledperson and therefore will not be described more thoroughly here. Theaforesaid layers are doped to the desired impurity content with suitablep-dopants such as Zn, C or Mg or with suitable n-dopants such as Te, Se,S and Si, respectively, as known in the art.

The active semiconductor layer sequence 14 can alternatively comprise amultiquantum well structure, as also known, for example, from the priorart.

Applied to the p-type AlGaInP confining layer is a thicklight-decoupling layer 16 of (Ga_(x)(In_(y)Al_(1−y))_(1−x)P, wherein0.8≦x and 0≦y≦1, or of GaP. Since the bandgap of the decoupling layer isgreater than that of the active layer, light-decoupling layer 16 istransparent to electromagnetic radiation generated in active layersequence 14.

In the case illustrated, the necessary current for powering the LED chipis supplied to the active layer of the LED chip 10 via a front-sidecontact 18 and a back-side contact 20. However, the contacts canalternatively be arranged otherwise than as explicitly shown in theembodiment example.

Light-decoupling layer 16 is applied by organometallic vapor-phaseepitaxy (OMVPE). Tertiary butyl phosphine (TBP,(C₄H₉)PH₂) is used as aphosphorus source and trimethyl gallium as a gallium source, and a V:IIIflux ratio of about 10 is selected. The growth temperature in theembodiment example is 720° C., a temperature at which the germaniumsubstrate is still sufficiently solid in the reactor.

In the embodiment example, layer sequence 14 has a cross section of 250μm×250 μm and a layer thickness of between 2 and 10 μm.

The features of the invention disclosed in the foregoing description, inthe drawing and in the claims can be essential to the practice of theinvention both individually and in any combination.

1. A method of fabricating a radiation-emitting semiconductor chip basedon AlGaInP, comprising the method steps of: preparing a substrate;applying to said substrate a semiconductor layer sequence comprising aphoton-emitting active layer; and applying a transparent decouplinglayer, wherein said substrate is formed substantially of germanium andsaid transparent decoupling layer is applied in a temperature rangeextending no higher than 800° C.
 2. The method as described in claim 1,wherein said transparent decoupling layer is applied with the use oftertiary butyl phosphine as a phosphorus source.
 3. The method asdescribed in claim 1, wherein said transparent decoupling layer isapplied at a temperature below 780° C., preferably below 750° C.
 4. Themethod as described in claim 1, wherein said transparent decouplinglayer is applied at a temperature of about 700° C.
 5. The method asdescribed in claim 1, wherein said transparent decoupling layer isapplied with the use of trimethyl gallium as a gallium source.
 6. Themethod as described in claim 1, wherein said transparent decouplinglayer is grown by organometallic vapor-phase epitaxy (OMVPE).
 7. Themethod as described in claim 2, wherein said decoupling layer comprisesGa_(x)(In_(y)Al_(1−y))_(1−x)P wherein 0.8≦x and0≦y≦1, particularly GaP.8. The method as described in claim 6, wherein said transparentdecoupling layer is grown with a V:III ratio of 5 to 20, preferably ofabout
 10. 9. A radiation-emitting semiconductor chip based on AlGaInPcomprising: a substrate; a semiconductor layer sequence applied to saidsubstrate and comprising a photon-emitting active layer; and atransparent decoupling layer disposed on said semiconductor layersequence, wherein said substrate is formed of germanium.
 10. Theradiation-emitting semiconductor chip as described in claim 9, whereinsaid transparent decoupling layer comprisesGa_(x)(In_(y)Al_(1−y))_(1−x)P wherein 0.8≦x and 0≦y≦1, particularly GaP.11. The radiation-emitting semiconductor chip as described in claim 9,wherein said transparent decoupling layer has a thickness of betweenabout 1 μm and about 10 μm, particularly of between about 2 μm and about10 μm.